1. Field of the Invention
The present invention relates to an image forming apparatus, for example, of electrophotographic type wherein an image is formed by transferring a toner image formed on an image bearing member onto a recording material (transfer material) born on a recording material bearing member.
2. Related Background Art
FIG. 2 shows an example of a color image forming apparatus for forming a full-color image.
In this example, a color image forming apparatus comprises a photosensitive drum (image bearing member) 1 supported for rotation in a direction shown by the arrow. Around the photosensitive drum 1, there are arranged a pre-exposure lamp 11, a corona charger 2, an optical system 3, a developing means 4, a transfer device 5 and a cleaning device 6.
The optical system 3 comprises an original scanning portion and a color decomposing filter. For example, the optical system is a laser beam exposure device for illuminating a color-decomposed light image or equivalent light image E onto the photosensitive drum 1. A latent image is formed on the photosensitive drum by illuminating the color-decomposed light image for each color onto the photosensitive drum 1 which was previously charged uniformly by the corona charger 2. The developing means 4 is rotatable and comprises four developing devices (i.e., black developing device 4Bk, cyan developing device 4c, magenta developing device 4m and yellow developing device 4y) arranged around a rotary shaft 4b so that a selected developing device can be rotated at a developing station where the selected developing device is opposed to the photosensitive drum 1, in order to develop the latent image on the photosensitive drum 1 with negative toner including resin as a main component, thereby forming a toner image.
Further, the toner image formed on the photo-sensitive drum 1 is transferred onto a recording material sent, by a convey system (through a convey path shown by the broken line in FIG. 2), from a recording material cassette 7 to a transfer station where the transfer device 5 is opposed to the photosensitive drum 1. In the illustrated example, the transfer device 5 comprises a transfer drum 5a, a transfer charger 5b, an absorb corona charger 5c for electrostatically absorbing the recording material and an absorb roller 5g opposed to the absorb corona charger, an inner corona charger 5d, an outer corona charger 5e, and an outer corona charger 5h. A peripheral opening of the transfer drum 5a supported for rotational movement is covered or closed by a cylindrical recording material bearing sheet 5f made of dielectric material.
As the transfer drum 5a is rotated, the toner image formed on the photosensitive drum 1 is transferred onto the recording material born on the recording material bearing sheet 5f, by means of the transfer charger 5b. A desired number of color toner images are transferred to the recording material born on the recording material bearing sheet 5f, thereby forming a full-color image. After the desired number of toner images were transferred to the recording material, the recording material is separated from the transfer drum 5a by a separation means 8, and the separated recording material is discharged onto a tray 10 through a heat roller fixing device 9.
On the other hand, after the transferring operation, the residual toner remaining on the photo-sensitive drum 1 is removed by the cleaning device 6 for preparation for next image formation.
In the past, as shown in FIGS. 7A to 7F, in order to erase the history of the photosensitive drum 1 before the copying operation is started, the photo-sensitive drum 1 was rotated in a direction shown by the arrow until the rotation of the drum was stabilized. Then, the residual charge was removed from the photosensitive drum 1 by energizing a pre-exposure lamp 11 (FIG. 7A), and then, the charge-removed area of the photosensitive drum 1 was uniformly charged by the charger 2 (FIG. 7B). After the charged area was subjected to the electricity removal be means of the pre-exposure lamp 11 (FIG. 7D) and the charging by means of the charger 2 (FIG. 7E) again, the latent image was formed on the photosensitive drum (FIG. 7F). Now, an operation effected from the input of an image information start signal (from an external device) to re-electricity removal on the area of the photosensitive drum once charged by the charger 2 is referred to as a "pre-rotation".
Here, it is considered that a full-color image is formed without performing the conventional pre-rotation. That is to say, as shown in FIGS. 8A to 8C, the photosensitive drum 1 is rotated in a direction shown by the arrow until the rotation of the drum is stabilized. Then, after the pre-exposure lamp 11 is energized and the photosensitive drum is charged by the charger 2, the formation of the latent image is started during the same revolution of the photosensitive drum 1.
By starting the charging by means of the charger 2 and the latent image formation by means of the optical system 3 during the same revolution of the photosensitive drum 1, a time period from the start of the charging by means of the charger 2 to the start of the latent image formation can be reduced in comparison with the conventional example shown in FIGS. 7A to 7F by a time period corresponding to one revolution of the photosensitive drum 1. However, if the pre-rotation is omitted, areas of the photosensitive drum 1 other than the area charged by the charger 2 are rotated while contacting with the recording material bearing sheet 5f in a condition that they are not charged by the charger. That is to say, when a recording material bearing surface of the recording material bearing sheet 5f is charged positively and the rear surface of the recording material bearing sheet is charged negatively with a polarity the same as that of the charger 2, since the negative (minus) charge accumulated on the recording material bearing sheet 5f flows through the transfer charger 5, an area which is charged positively is created on the photosensitive drum 1 which is electrically earthed.
As shown in FIGS. 9A to 9C, if there are positive (plus) potential and negative (minus) potential on the photosensitive drum 1, even after the photosensitive drum is subjected to the electricity removal (for making the minus potential to zero potential) effected by the pre-exposure lamp 11, the plus potential is not removed (FIG. 9B). Thus, in the first color image formation, there arises a problem that the positively charged areas on the photosensitive drum affect a bad influence upon the image (FIG. 9C). In particular, a half-tone image of the hi-light portion is strongly affected by the history of the plus charge.